Numerical simulation of wall dynamics in (111)-oriented garnet films in the presence of an in-plane magnetic field

The domain wall motion in the presence of an in-plane magnetic field H_y perpendicular to the wall is simulated using a fall implicit numerical scheme. Calculations are performed for the drive fields 0 Oe<H_z<15 Oe and in-plane fields -210 Oe?H_y?210 Oe. The relation between the average wall velocity $\text{v}$ and the drive field H_z is discussed considering the wall structure. It was found that an in-plane field increases the peak velocity of the wall and extends the range of the drive fields, where the linear mobility relation is valid. A dynamical Bloch line stacking was found for sufficiently large drives. The influence of an in-plane field on the angular span of horizontal Bloch lines is discussed also. In particular the occurrence of 2π-horizontal Bloch lines is described. Numerical results obtained with a full implicit method are compared with the experimental observations of bubble motion and good agreement is found for |H_y|≤100 Oe.     

Switching field dependence on heating pulse duration in thermally assisted magnetic random access memories

The minimum applied field H_SW required to reverse the magnetic moment of the ferromagnetic/antiferromagnetic storage layer of a thermally assisted magnetic random access memory (TA-MRAM) device during the application of a heating electric pulse is investigated as a function of pulse power P_HP and duration δ. For the same power of the heating pulse P_HP (or, equivalently, for the same temperature of the storage layer), H_SW increases with decreasing heating time δ. This behavior is consistently interpreted by a thermally activated propagating domain-wall switching model, corroborated by a real-time study of switching. The increase of H_SW with decreasing pulse width introduces a constraint for the minimum power consumption of a TA-MRAM where writing combines heating and magnetic field application.     

Dependence of the domain-wall velocity on the driving field in garnet films

The dependence of the domain-wall velocity on the amplitude of the driving magnetic field pulses is investigated in an iron garnet film of the (YSmCa)₃(FeGe)₅O₁₂ system with a (111) orientation. The results obtained are analyzed from the standpoint of existing theory. A maximum corresponding to the disruption of steady-state motion is observed on the dependence. Thereafter, the velocity at first decreases sharply and then increases. It is theorized that a process involving the periodic generation, propagation, and annihilation of horizontal Bloch lines occurs in the wall in this period. Data are obtained for the velocity saturation region, which confirm a previously proposed empirical formula and a theoretical model, according to which the saturation regime corresponds to a state of chaos. Fiz. Tverd. Tela (St. Petersburg) 39, 660–663 (April 1997)     

Dynamics of domain walls in bismuth-ytterbium-containing garnet ferrite films in the vicinity of the angular momentum compensation point

The dependence of the domain-wall velocity V on the acting magnetic field H is investigated for (Bi,Yb)₃(Fe,Ga)₅O₁₂ garnet ferrite single-crystal films in the vicinity of the angular momentum compensation point at different temperatures. The films are grown by liquid-phase epitaxy from a supercooled solution melt on Cd₃Ga₅O₁₂ substrates with the (111) orientation. It is demonstrated that, in these films, the precessional mechanism is not responsible for the motion of domain walls but there arises an internal effective magnetic field that weakens the acting magnetic field.     

原子运动和场模结构对原子动力学特性的影响

研究了双光子Jaynes-Cummings(J-C)模型中原子的动力学特性,讨论了在不同初态下,原子运动和场模结构对体系中原子反转的演化特性及原子偶极矩k阶压缩的影响.研究结果表明,原子的动力学特性不仅决定于原子和场的初态参量,而且通过时间因子g^Θ(t)=2^-1gt-(4_p)^-1sin(2_pgt)决定于原子的运动速度与场模结构参量.     

Dynamics of domain walls in magnetically multiaxial films in the thickness range corresponding to the Bloch-Néel transformations of the structure

The numerical minimization of the total energy functional and the solution of the nonlinear Landau-Lifshitz equation have been performed exactly taking into account the fundamental (including dipole-dipole) interactions in terms of the two-dimensional magnetization distribution. The equilibrium structure, energy, mobility, and scenario of the dynamic transformation of the domain walls (in their non- steady-state motion) have been determined as a function of the film thickness b and external magnetic field H for two different ((010) and (110)) orientations of the surfaces of magnetically triaxial films. The range of film thicknesses, including the thickness b = b _N, for which the Néel domain walls can be transformed into the Bloch domain walls, has been investigated. The phenomena of anisotropy of the domain-wall energy, the domain-wall mobility, and the period of dynamic transformations of the domain walls have been analyzed as a function of the film thickness b and external magnetic field H. The range of film thicknesses has been determined, in which the non-steady-state motion of the Néel domain walls is accompanied by the creation and annihilation of vortex-like structures despite the one-dimensional character of the magnetization distribution in these walls.     

The defects influence on domain wall propagation in bistable glass-coated microwires

We studied the domain wall (DW) dynamics of magnetically bistable amorphous glass-coated Fe₇₄B₁₃Si₁₁C₂ microwires. In according to our experimental results magnetic field dependences of DW velocity of studied microwires can be divided into two groups: with uniform or uniformly accelerated DW propagation along the microwire. Strong correlation between the type of the magnetic field dependence of domain wall velocity, v(H), and the distribution of the local nucleation fields has been observed.Moreover, we observed abrupt increasing of DW velocity (jump) on the magnetic field dependences of the domain wall velocity, v(H), for the both types of the v(H) dependences. At the same time usual linear increasing of the domain wall velocity with magnetic field persists below these jumps. It was found that the jump height correlates with the location of nucleation place of the new domain wall. We have measured local nucleation field distribution in all the microwires. From local nucleation field distribution we have obtained the DW nucleation locations and estimated the jump height     

Classical dependence of the domain wall velocity on the magnetic field in garnet ferrite films with orthorhombic magnetic anisotropy

The dependence of the domain wall velocity V on the acting magnetic field H is investigated for bismuth-containing single-crystal garnet ferrite films with orthorhombic magnetic anisotropy. It is shown that this dependence includes both the initial linear portion and a saturation portion and exhibits a complex behavior. This behavior is explained within the model of domain wall motion with spin wave radiation.     

Stress dependence of the domain wall dynamics in the adiabatic regime

In this work, we determine the domain wall velocity in the low field region and study the domain dynamics in as-cast and annealed bi-stable amorphous glass-covered Fe_77.5Si_7.5B₁₅ microwires. In particular, from the relation between the domain wall velocity and magnetic field in the adiabatic regime, the power-law critical exponent β, the critical field H₀ and the domain wall damping η were obtained. It has been verified that the main source of domain wall damping is the eddy current and spin relaxation, both with a strong relation with the magnetoelastic energy. This energy term is changed by the axial applied stress, which, by its time, modifies the damping mechanisms. It was also verified that the domain wall damping terms present different behavior at low (mainly eddy currents) and high applied stress (spin relaxation).     

Phase transitions at high pressure in the fullerene C60 molecular donor-acceptor complex {Hg(dedtc)2}2 · C60

Raman spectra of crystals of the molecular fullerene C₆₀ donor-acceptor complex {Hg(dedtc)₂}₂ · C₆₀ (fullerene with mercury diethyldithiocarbamate) have been measured at a pressure up to 8.4 GPa and at room temperature. A phase transition has been revealed in the pressure range of 1.2–2.0 GPa, which is accompanied by a splitting of the degenerate intramolecular phonon modes H _g (1)-H _g (4) and H _g (7)-H _g (8), as well as by a softening of the H _g (2) mode of the fullerene C₆₀. As the pressure further increases to the maximum value, the intensity of the bands varies smoothly. A decrease in the pressure leads to the reverse transition to the initial state at 1.2 GPa. The splitting of the degenerate modes H _g (1)-H _g (8) and the softening of the H _g (2) modes resemble their behavior in the formation of dimers in fullerite crystals and indicate the possible formation of dimers in two-dimensional fullerene layers under hydrostatic compression of the {Hg(dedtc)₂}₂ · C₆₀ complex.     

90° pulsed magnetization of ferrite-garnet films with easy-plane anisotropy

The process of pulsed 90° magnetization of ferrite-garnet films was studied. These films, in addition to easy-plane anisotropy, have biaxial anisotropy in the film plane with an effective field H _K2 ? 40–55 Oe. the pulsed magnetization curve contains two portions separated by a kink observed at a field pulse amplitude H _p=H _p ^* ? 16–18 Oe. An analysis of the magnetization signals showed that the restoring torque, which is mainly caused by biaxial-anisotropy forces, is overcome in fields H _p ≥ H _p ^* and that magnetization rotation occurs. In fields H _p < H _p ^* , the magnetization vector rotates at the initial stage only and the angle of rotation ?_in is less than 25°–26°. The field H _p ^* and angle ?_in are calculated. The results of the calculations are confirmed by experimental data. In fields H _p > H _p ^* , the process of magnetization is accompanied by oscillations of the magnetization vector. In contrast to free magnetization oscillations, these oscillations are nonlinear and the frequency of the first harmonic (≈5 × 10⁸ Hz) is much lower than that for free oscillations, (7–12) × 10⁸ Hz. Oscillations are excited at a pulse rise time of ≈6 ns.     

Second-harmonic generation taking into account dispersion of nonlinear susceptibility

Second-harmonic generation in the field of an ultrashort pulse and the propagation of extremely short pulses in a medium with quadratic nonlinearity are analyzed. Second-harmonic generation is analyzed taking into account the effect of second-and third-order group velocity dispersion and dispersion of nonlinear susceptibility up to the second order. Corrections, whose order of smallness is determined by the parameter (ω_L t _p)^?1, where t _p is the pulse duration and ω_L is the carrier frequency of the pump wave, are obtained. For a large phase mismatch, two new solutions are found that describe the stationary evolution of solitary pump and second-harmonic waves in the regions of both anomalous and normal group velocity dispersions.     

On the limiting velocity and forced motion of ferromagnetic domain walls in an external field perpendicular to the easy-magnetization axis

A theory is constructed for the dynamics and braking of domain walls in ferromagnets when a magnetic field is applied perpendicular to the axis of easy magnetization (i.e., a transverse field H _⊥). The theory is valid for velocities v up to the limiting domain wall velocity v c. The Landau-Lifshitz equations in the dissipationless approximation are used to investigate the motion of domain walls and the change in the character of the wall motion as its velocity v approaches v _c. The force acting on a domain wall due to viscous friction is calculated within the framework of generalized relaxation theory, and the dependence of the domain wall velocity v on the forcing field H _z is investigated. Calculations of the braking force show that the contributions of various dissipation mechanisms to the friction force have different dependences on the domain wall velocity, which affects the form of the function v=v(H _z). The shapes of the curves v(H _z) differ very markedly from one another for different values of the field H _⊥. The theory developed here can be used to describe the experimental results, in particular the almost linear behavior of v=v(H _z) for small H _⊥ and its strongly nonlinear behavior when H _⊥∼H _a, whereas these data cannot be reconciled within the standard theory based on relaxation terms of Hilbert type. Zh. éksp. Teor. Fiz. 112, 953–974 (September 1997)     

Spin drift velocity, polarization, and current-driven domain-wall motion in (Ga,Mn)(As,P)

Current-driven domain-wall motion is studied in (Ga,Mn)(As,P) ferromagnetic semiconducting tracks with perpendicular anisotropy. A linear steady state flow regime is observed over a large temperature range of the ferromagnetic phase (0.1T(c)相似文献   

Phase front motion in uniaxial ferromagnet

It is well known that in the uniaxial ferromagnet in the presence of an external magnetic field perpendicular to the easy axis (h_x) a continuous phase transition occurs for a critical value of this field. There are metastable and stable states if one includes a small field parallel to the easy axis (h_z). The motion of the relaxation front of the metastable state is investigated. It is found that an “interphase wall of Neel-type” exists, its velocity is proportional to h_z and increases when the critical point is approached.     

Nonlinear dynamics of domain walls with a vortex internal structure in magnetouniaxial films with planar anisotropy

The nonlinear and generally unsteady dynamics of domain walls with a vortex internal structure in a constant magnetic field H is investigated on the basis of the numerical solution of the Landau-Lifshitz equation for a 2D distribution of magnetization M in magnetic films with planar anisotropy taking into account exactly the main interaction, including the dipole-dipole interaction. It is shown that in addition to field H _c (bifurcation field) above which the motion of a wall becomes unsteady and its internal structure experiences global dynamic changes, there exists a field H₀ separating two steady motions of the wall with different structures. The data clarifying the physical origin of the nonlinear dynamic rearrangement of the wall structure are presented. New rearrangement mechanisms associated with the generation and attenuation of vortices as well as their tunneling through the central surface of the wall are established. The existence of subperiod oscillations of the wall velocity in a static field in addition to the oscillations associated with the precession of M around the easy magnetization axis is predicted. The period T of dynamic variations of the wall structure is studied, and an empirical formula is proposed for describing the singular behavior of the T(H) dependence near H=H _Hc with the critical index depending on the film parameters. The bifurcation process is studied, and a nonlinear dependence of the critical field H _c on the film thickness and the saturation magnetization is established. The possibility of direct experimental investigation of the dynamic rearrangement of the internal structure of the wall is indicated.     

Effect of the magnetic field in the plane of a garnet ferrite film with orthorhombic magnetic anisotropy on the dynamics of domain walls

The effect of an in-plane magnetic field on the dependence of the domain-wall velocity on the acting magnetic field is investigated for bismuth-containing garnet ferrite single-crystal films of the composition (Bi,Y,Pr)₃(Fe,Ga)₅O₁₂ with the (210) orientation. The in-plane magnetic field is applied along the 〈120〉 and 〈001〉 crystallographic axes. The domain-wall velocity is measured in directions perpendicular and parallel to the in-plane magnetic field.     

Magnetic field dependence of asymmetry in the magnetization reversal of ultrathin Co films and Co/Pt multilayers with perpendicular anisotropy

We studied the magnetization reversal in ultrathin [Co/Pt]_n films (n=1, 2, and 4) using magneto-optical Kerr microscopy. These materials demonstrate unusual asymmetries in the activity of nucleation centers and domain wall motion. It was found that application of very high holding magnetic field prior to magnetization reversal, exceeding some critical value much larger than the apparent saturation field, suppresses the subsequent ‘asymmetric’ nucleation centers, activity. We revealed that the ‘asymmetric’ nucleation centers become active again after subsequent reversal cycles coming from a smaller holding field and studied how the asymmetry returns with the decrease of applied holding field. It was found that in low-coercivity ultrathin Co films, the asymmetry in domain wall velocity decreased sharply with the applied field increase and disappeared when the reversal field is greater than μ₀H=1.5 mT.     

Optimal laser pulse design for transferring the coherent nuclear wave packet of H+2

Within the Franck–Condon approximation, the single ionisation of H₂ leaves H⁺₂ in a coherent superposition of 19 nuclear vibrational states. We numerically design an optimal laser pulse train to transfer such a coherent nuclear wave packet to the ground vibrational state of H⁺₂. Frequency analysis of the designed optimal pulse reveals that the transfer principle is mainly an anti-Stokes transition, i.e. the H⁺₂ in 1sσ_g with excited nuclear vibrational states is first pumped to 2pσ_g state by the pulse at an appropriate time, and then dumped back to 1sσ_g with lower excited or ground vibrational states. The simulation results show that the population of the ground state after the transfer is more than 91%. To the best of our knowledge, this is the highest transition probability when the driving laser field is dozens of femtoseconds.     

Formation of the space charge region in diffusion p-n junctions under high-density current interruption

The recovery of diodes with diffusion p-n junctions in the case of high reverse current density j is analyzed. A condition for quasi-neutrality breaking in the diffusion layers with allowance for the dependence of charge carrier mobility μ on electric field strength E is obtained that is valid for a wide range of j. The problem of formation of the space charge region in a circuit with inductance L and resistance R is reduced to a system of two ordinary differential equations. Approximation of a numerical solution to this system makes it possible to derive crude analytical relationships between interrupted current density {ie88-1}, circuit parameters, diode parameters, and parameters of a forming voltage pulse (with amplitude V _m and pulse rise time t _p). The limiting parameters of a pulser with an inductive energy storage and current interrupter based on diffusion diodes are studied. The critical density of interrupted current {ie88-2} is determined at which the field in the space charge region near the anode reaches breakdown value E _b and intense impact ionization by holes begins. The impact ionization decreases the rates of current decay and voltage increase in the space charge region. As a result, at {ie88-3}, t _p starts increasing and the overvoltage factor of the pulser decreases. The value of V _m corresponding to {ie88-4} is roughly given by {ie88-5}, where m is the number of diodes in the interrupter, ? is the permittivity of the semiconductor, {ie88-6} is the saturated drift velocity of holes, and l _p is the depth of the p-n junction (diffusion depth). Theoretical predictions are confirmed by exact numerical simulation of the recovery process and qualitatively agree with the available experimental data.